At 5am, before he begins his six-hour training session, Tim Reed tucks into a breakfast of four eggs, sausages, full-fat yoghurt and some nuts washed down with a "bulletproof" coffee (coffee with butter and coconut oil).

If he's doing a particularly hard session, he'll take along boiled eggs to snack on or stop for a sushi roll. Dinner is often meat with a cream sauce and a salad.

With this diet Reed, 29, says he doesn't experience "the bonk", the moment when a professional athlete goes "from flying to dying". Rather, he slows down gradually.

Over the past five years, as he started to see the benefits of avoiding sugar and processed carbohydrates, he moved into fat.

The decision to eat more fat flies in the face of accepted wisdom for endurance athletes, who have long been encouraged to use carbs for fuel. But Reed is part of what appears to be a push, among some endurance athletes, away from carbs.

Becoming "fat adapted" takes time and can be a demanding process. "I found my blood pressure dropped in general and I had to increase my sodium intake to counteract that," says Reed. "You feel really sluggish at times and for quite a few weeks you feel hungry all the time.

"When I really started eliminating carbohydrate my body was really craving it like my body was addicted to the carbohydrate. It took me three weeks of feeling quite bad and then you start to adapt and then by six months you start to feel really great."

Reed hasn't completely abandoned carbs and uses them on race days.

WINNING EDGE

"Race day nutrition changes according to the distance," he says. "Typically I use maltodextrin-based gels and MCT oil as the primary fuel. There is not a lot healthy about race day, it's all about short-term maximum performance."

While using fat as fuel may seem counter-intuitive, there is good evidence of its effectiveness in endurance sports.

The recent popularity of low-carb diets among such athletes is widely attributed to South African Tim Noakes, professor of exercise and sports science at the University of Cape Town and a marathon and ultramarathon runner.

Noakes switched to a low-carb diet as a way to lose weight and combat diabetes. The results were so good, Noakes now evangelises the benefits of low-carb, high-fat eating for everyone, not just endurance athletes. Among the benefits, he says, is that people don't feel hungry: "If you're hungry you'll never control your weight," he told paleorunner.org in 2013. He believes the benefits for athletes go much further than weight control: "For events of more than five hours, fat-adapted athletes have an advantage," he said.

THE FAT UNDERTAKER

There's nothing new about low-carb, high-fat (LCHF) diets. In the 1800s low-carb diets were used to treat diabetes but it was an overweight London undertaker called William Banting who popularised the diet when he successfully used it to lose weight.

Banting was so delighted by the results, prescribed by his doctor William Harvey, that in 1864 he published Letter on Corpulence Addressed to the Public to spread the word about the benefits of avoiding carbs. And 30 years ago, researcher Stephen Phinney found that experienced endurance cyclists on a low-carb diet performed as well as on high carbs but were able to conserve glucose and muscle glycogen (the fuel needed for high-intensity sprinting) because they were burning fat when they weren't pedalling flat out.

Low-carb diets periodically still become popular among the general public as a way to shift kilos. Recent variants of these diets include the Atkins of the 1980s, South Beach of the 1990s and today's paleo.

THE RISKS

But LCHF diets are not universally lauded. On the Sports Dietitians Australia website, it says: "There is no clear evidence that fat-adaptation strategies offer any benefits for the endurance athlete over the traditional high-carbohydrate diet."

Other health professionals warn of risks such as an elevation in inflammatory gut bacteria, bone mineral loss, a rise in blood lipids (which is linked to heart disease) and fatigue. There is increasing debate about whether cholesterol is a cause of heart disease but the proponents of the LCHF diet, such as Grant Schofield of Auckland University of Technology, say blood fats are unaffected.

"As long as you [increase the amount of fat you eat] in the context of reducing your carbohydrates, it ends up quite well. Contrary to popular opinion, all the things we consider health risks, your blood fats and blood pressure also improve as well."

While a low-carb diet may be good for everyone according to the advocates, it is not going to confer any advantage to short-course athletes. Its uptake among the endurance crowd has not been quantified and unlike the evangelising scientists, pro athletes are more taciturn. "So many pro athletes don't go on about [the diet] because they don't want to share it with the athletes that don't do it," Reed says. "But among the athletes I know, 40 to 50 per cent are training their body to use fat for fuel."

THE ENDURANCE ATHLETE CASE STUDY

Grant Schofield, professor and director of the Human Potential Centre at Auckland University of Technology, first became interested in high-fat diets when he was working in the Pacific.

"You'd walk round with your World Health Organisation manual going eat less fat and exercise more and in the outer Pacific Islands they eat quite a lot of fat but they're quite lean and there's no chronic disease or diabetes and you'd think, 'That's odd, it's the opposite of what we've been told.' Then you start to think a bit more about fat and carbohydrate and metabolism."

More recently he has been studying the effects of a low-carb, high-fat diet on endurance athletes.

Schofield explains that when people rest or exercise moderately, they burn fat. When they exercise hard, their bodies switch to carbohydrates.

But eating carbs causes their insulin to rise which, in turn, stops the burning of fat. When they run out of glucose (from carbs), they have nothing left to power their muscles and can't keep going.

By switching to a low-carb diet, the body becomes fat-adapted and burns fat for fuel when it runs out of glucose.

To judge the effects of diet on performance, Schofield has been subjecting New Zealand triathlete Bevan McKinnon to a battery of tests.

One of these is the VO2, which measures how much oxygen Bevan uses when he is flat out. By analysing his breath during this test, it possible to tell if he is burning fat or carbs to power his muscles.

"Before [McKinnon] was on the diet, at the point where he was burning 50 per cent fat and 50 per cent carbs, he was able to generate 130 watts on an exercise bike. Twelve weeks [after he switched to a LCHF diet] he was able to produce 330 watts with the same fuel mix efficiency," says Schofield.

In studying the use of carbs in conjunction with a LCHF diet, Schofield initially thought a fat-adapted athlete who used carbs when exercising hard would drive insulin up enough to reduce fat burning and so negate the whole effect.

But that didn't happen.

"If athletes are going hard enough, they dispose of the carbs without jacking up insulin too much," he says.